Are there any fundamental problem with the designs of MiG-29 and SU-27? Why do they tend to underperformed(Newer SU-27 variants have less payload than equivalent F-15 variants etc.),Or is it more about lack of updates(too little composite,Outdated engines tech,Subpar avionics etc.)
My guess is going to be avionics and engine tech. Older avionics will limit targeting range, while engines will affect lifting potential. Note that these issues will affect Ukraine and Russia both with these legacy platforms. Note also that these may be upgraded without too much turbulence.
Put together, lower payload with less range, or perhaps shorter TBOs, that might give them both maintenance issues, without those upgrades.
The designs themselves seem pretty capable as airframes, though I think the -29 might have range issues?